Game Ranging / Field Guiding Course. Savanna Ecology

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1 1 Module # 10 Component # 7 Introduction To fully understand life on the African plains, a comprehension of is possibly the most crucial topic to come to grips with. This component will attempt to introduce you to the in-depth workings of these intricate systems. It is generally accepted that ecological diversity is the best insurance against habitat deterioration. The primary aim of conservation must lie with maintaining the greatest possible biodiversity. From a conservation point of view, the question of how to optimise ecosystem functioning for maximising biodiversity is of paramount importance. However, the complexity of the relationships between biological diversity and ecosystem function, the task of establishing effective strategies is not easily achieved. A common misconception is that biodiversity is synonymous with species richness or any other numerical descriptor of organisms, but biodiversity can be divided into three components: composition, structure and function. In Southern Africa, savannas range from tall moist woodlands to sparse grasslands with scattered thorn bushes in arid to semi-arid areas.

2 2 Savanna Ecosystems Describing African savannas is not a simple task, with various scientists putting forward several ideas on the subject. In short, the savannas of Africa are characterised by having a continuous, well-developed grass layer and an open, discontinuous layer of shrubs or trees. Savannas owe their structure primarily to the complex interaction of soil, water, vegetation and nutrient availability with fire and herbivores acting as the agents that mold the specific characteristics and architecture of each specific system. We recognize that although large reserves may look like natural systems, they are in fact quite artificial, since it was us that defined the boundaries. Thus, since we have created these systems, it is our responsibility to manage them as best we can. The major structural components of these systems are largely out of the control of managers of the system, we cannot control rainfall, climate, plant growth or nutrients, but we can have a significant effect on the ecosystem by manipulating fire and herbivores.

3 3 African Savanna Ecosystems :- Major Components Water Annual primary production, or how much growth occurs in these ecosystems, is very variable and closely follows the pattern of rainfall. Although rainfall is demonstrably a dominant control factor for plant growth, it is in fact the seasonal differences in soil-moisture which directly influences this important factor. Soil water availability controls the duration of plant growth (primary production) and nutrient availability. Nutrients Plants require nutrients in different amounts. Those required in relatively large amounts are termed macronutrients, and those required in smaller amounts are termed micronutrients. For plants to take up nutrients from the soil, the nutrients must be: Present In an inorganic form In the soil solution (i.e. dissolved in soil water) Macronutrients: Nitrogen and phosphorous are essential for the growth and functioning of all organisms. Their concentration in plant and animal tissues must fall within a specific range. Their availability is a potential constraint on productivity. Nitrogen and phosphorous are the most frequently limiting nutrients in terrestrial ecosystems, and savannas are no exception. Micronutrients: Calcium, magnesium and sodium are not generally thought to be important limiting nutrients in dry savannas, but the situation could be different in a moist savanna. This group of elements, are lost mainly from the system by soil water drainage in a process known as leaching.

4 4 Herbivores It is widely accepted that the current look of savanna ecosystems is the result of co-evolutionary adaptation (several factors influencing each other) between vegetation and animals. Savannas of South and Central Africa evolved under light and probably intermittent utilization (grazing and browsing) by mixed communities of wild (as opposed to domesticated) herbivores. These savannas are adapted to seasonal relief from grazing and browsing pressure. The reason for the seasonal relief was that animals would have had to either move off the grasslands in search of water, or the grasslands themselves were highly variable in terms of their nutrition from season to season. If, however, the ecosystem did not receive any significant relief from grazing, certain plant species would have become locally extinct and combined with the effects of drought and soil degradation, whole ecosystems would have deteriorated. Acceptability and availability of forage (food) For plant material to be acceptable to herbivores to eat, it must be both nutritious and digestible. Large mammalian grazers face a period of protein shortage during the dry season, and browsers face a period of energy shortage at this time. Vegetation (leaves and trees) consumed by browsers has a high protein content relative to grass, even in the dry season, but owing to the high proportion of deciduous plants (trees that lose their leaves in winter) in savannas, forage (leaves) is seasonally scarce. At high stocking rates, grazers could be under both protein and energy stress. This is because grass nutrition levels drop during the late dry season and essential nutrients are lost out of grass at this time. It has been found that the plant species with the highest nutrient content show the greatest seasonal variation. In large reserves the game population should include both a wide diversity of different grazers and browsers and therefore it would be detrimental toward certain species to manage the system towards maintaining a dominant and uniform vegetation structure. In other words, livestock management criteria cannot be adopted for the management of wildlife. Habitats selected or avoided by herbivores will be significantly different from habitats utilised in proportion to their availability. Body size has a major impact on feeding behaviour of browsers, not only in the maximum height at which they can feed, but also in the type of material they can select. Mouth size is important for feeding since it determines how much vegetation can be eaten (ingestion rate) and therefore efficiency of digestion. Body size also controls metabolic requirements of animals. Smaller animals require more protein in their diet. The smallest browsers are therefore forced to pick at only the highest quality plant parts, while large browsers can afford to be less selective.

5 5 Fallen leaves and pods are important in the diets of mixed feeders (herbivores that both graze and browse). This could have important implications for the reintroduction of certain species. Browsers in certain specific habitats (E.g. broadleaf savanna) have a further problem in that most of the plants contain chemicals that hinder digestion. Food scarcity in winter forces them to eat unpalatable species. Grazers with small mouths may be selective, but can utilise habitat that excludes herbivores with larger mouths because they cannot select for the favourable components in the tall grass. In reality, all grazers are as selective as the size of their mouthparts allows them to be. This is particularly evident in nutrient limited savannas where minimum maintenance of protein and digestibility are important.

6 6 Plant Defences against Herbivores Trees Plants on infertile savannas are slow-growing, and cannot afford to lose nutrients or leaf tissue to herbivores. One defence against herbivory (the eating by herbivores) is to store nutrients underground. There is a class of woody plants which take this trend to the extreme, with only the leaves being exposed at ground level. The consequence of this defence mechanism, however, is strong competition with grasses for light. However, with water and nutrients stored underground, they can grow during the dry season. This makes them very susceptible to herbivory, since grass resources are limited at this time, and fresh growth would be very appealing. Hence the second form of plant defence: noxious chemicals. Certain trees have developed the ability to manufacture a range of highly unpalatable chemical compounds, specifically tannins and phenols. These substances, known as secondary chemicals are produced and stored in the leaves of the tree. They give the plant the characteristics of being both highly unpalatable and highly indigestible. The trees don t necessarily continually maintain high levels of these chemical compounds within their leaves. One study on the feeding habits of Kudu showed that the tree it fed on Acacia caffra (Common hook-thorn), increased its tannins levels some 94% within a few minutes of being fed on. There is also evidence to suggest that one plant can communicate to others around it to begin to increase their secondary chemical production due to herbivores in the vicinity. This action by the plants result in preventing overbrowsing. Not only do these chemicals make the leaves unpalatable, but even when consumed they are indigestible. In one specific study, it was eventually discovered that kudu were dying from malnutrition with full stomachs. This was due to the forage that was consumed being completely indigestible. Grass The location of the meristem (growth tissue) is an important adaptive defence against herbivory for grasses. When it is at the tip of the branch, as it is for trees, if it is eaten, then the capacity of that branch to generate new growth is removed. With it being located at the base of the tuft, near the ground, grasses can produce new leaves close to the ground and the loss of leaf material does not prevent the growth of new leaves. Hairs on the leaf surface on both trees and grasses are an important deterrent mainly to insect herbivores. Hairs represent a structural barrier to insects much like thorns do to mammals. Hairs also reduce water loss.

7 7 Fire Fire has been well established as a major factor that determines the structure of savanna vegetation. Savanna studies have shown that fire exclusion alone can alter the grass/tree balance in favour of trees. Sustained co-existence of trees and grasses is thought to depend on the action of fire across a landscape, with some facets favouring grasses and others favouring trees. Many savanna trees re-sprout as juveniles or revert to re-sprouting from the base if the canopy is burnt. In this stunted state, they compete with grasses and are more heavily browsed. Grasses are extremely effective at suppressing regeneration of other growth forms because of their rapid growth rates. Trees can be prevented from emerging from the grass layer by regular burning. If fires are infrequent or of low intensity, trees grow to the point where they overtop grasses, reducing the risk of future fire. If the preservation of biodiversity is dependent on the preservation of natural processes, then the use of fire in conservation management is essential. The fire regime refers to the type and intensity of fire and the season and frequency of burning. The current thinking is that a fixed burning program will inevitably lead to the impoverishment of diversity by repeatedly favouring the same species at the expense of others. When selecting a burning program for conservation, as is the case of most management options, there may be no optimum for maximising biodiversity.

8 8 Management of Savanna Environments A generally accepted principle in conservation management has been that the smaller the area being used for wildlife, the more intensively it must be managed. In smaller conservation areas, the most important factors to consider in the formulation of a management programme have been stated to be: Assessment of range (veld) condition Monitoring of range condition The setting of realistic stocking rates (game population numbers) of adapted wildlife species Grazing and browsing management Water provision Burning Culling and cropping Range condition Range condition refers to the condition of the vegetation in relation to a defined functional characteristic such as average plant production and resistance to soil erosion. There are certain features on a site (reserve) that can be assessed or measured to determine the range condition. There are a wide range of characteristics that influence whether a habitat is suitable or not for different game species. The development of a technique for assessing range condition in the Kruger Park has proven very popular for monitoring range condition in South Africa. The technique entails categorising grass species according to their responses to different intensities of grazing. Species which decrease in number when range is overutilised are termed decreasers. Species that increase when range is under-utilised are termed increaser I species. Increaser II species increase when range is overutilised. Variable species are a group of grasses that show variable responses to utilisation. Management is often directed towards improving range condition, and an expansion in decreaser-grasses is widely recognised as indicative of range in good condition. However, not all herbivores will benefit from range in good condition. Rather it is to the benefit of all herbivore species that range in different stages of utilisation should be present. If heavily utilised areas do not threaten the existence of any grazing species by becoming too large, or lead to exposed surfaces and accelerated soil erosion, heavily utilised areas should be regarded as an acceptable natural phenomenon.

9 9 What follows below is an example of a Veld Assessment, based on the grasses present in the area. This assessment was done at Combretum Park Nature Reserve. Grass type No. Samples Ecological status Palatability Succession stage Panicum maximum 3 Decreasers High Climax Trichoneura grandiglumis 1 Increaser 2 Low Sub climax Eragrostis pallens 1 Increaser 2 Low Climax Aristida congesta 6 Increaser 2 Low Pioneer - sub climax Digitaria eriantha 4 Decreasers Med Climax Eragrostis rigidior 1 Increaser 2 Low Sub climax Brachiaria brizantha 9 Increaser 1 Low Climax Aristida stipitata 2 Increaser 2 Low Pioneer Pogonarthria squarrosa 9 Increaser 2 Low Sub climax Melinis repens 7 Increaser 2 Low Pioneer - sub climax Perotis patens 17 Increaser 2 Low Pioneer - sub climax Urocholoa mosambicensis 1 Increaser 2 High Sub climax Cynodon dactylon 18 Increaser 2 High Pioneer Heteropogon contortus 1 Total 80 Increaser 2 Med Pioneer - sub climax SUMMARY Palatability: High 26/80; Medium 2/80, low 52/80 Desirability of species: Desirable Decreasers 3/14; less desirable increaser 2 s 10/14; undesirable increaser 1's 1/14 Veld is in a transition between sub-climax & climax. High proportion of grasses that indicate a history of disturbance. Recommended Management Strategy: Bring in a few selective grazers e.g. Blue Wildebeest or White Rhino for days. Allow resting for 6-8 weeks and repeating. Grasses utilised by selective grazers will be stimulated to grow and compete against the unpalatable.

10 10 Carrying capacity Numerous authors have attempted to define carrying capacity, but these definitions are not consistent. Determining carrying capacity is possibly the information most frequently requested by land managers. Invariably, carrying capacity attempts to: describe the productivity of the vegetation in terms of the number of animals that can be maintained in a productive state on an area of land without deterioration of vegetation or soil. The stocking rate of different game species on a game ranch will be determined by the types and condition of the different available habitats and management objectives. When game viewing is a priority, the stocking rate is usually maintained at or close to ecological carrying capacity, which is the maximum population of animals that an area can support without deterioration of range condition. The stocking rate that is maintained on a hunting reserve will have a very different population structure. Grazing and browsing management Continuous grazing and browsing generally leads to the selective overutilisation of the animal s preferred plant species. This usually results in a decline in the reproductive capacity of these species, and hence their ability to recover from these grazing and browsing disturbances. This can be prevented by rotational grazing and browsing. In small conservation areas where natural migrations of wildlife are not possible, management actions need to be taken to manipulate game movements. Burning Generally, the most effective way of attracting game to an area is to burn it. This practice results in the production of highly nutritious forage as a result of nutrients becoming readily available. The burning programme, however, must be carefully planned, with the most important factor being the size of area to be burnt. Care must be taken to ensure that the burnt area far exceeds the short term nutritional requirements of the game that will be attracted to it. Another important factor is that the stocking rate of grazers must be such that sufficient grass will accumulate for the implementation of a burning programme. If fire is excluded from a savanna, bush encroachment invariably results.

11 11 Water provision and Mineral licks Rotational resting of certain areas within a reserve can be implemented by: The strategic placing of mineral and salt licks The opening or closing of artificial waterpoints. In confined reserves in arid and semi-arid savannas, the provision of artificial water holes is necessary to sustain animal populations throughout the relatively long dry season. There are numerous instances, however, where the creation of abundant perennial (year-round) water points to enhance game viewing in these savannas has resulted in severe range degradation. This occurs because areas that could previously only be exploited by high-density, water-dependant herbivores during the wet season, become accessible to these animals during the dry season as well. This inflates short-term population numbers beyond an acceptable ecological level and leads to severe overgrazing. Culling The artificial confinement of herbivores characteristic of most Southern African reserves has resulted in unnaturally dense game populations. This contributes to overutilisation of reserve resources which results in the main justification for culling. The basis for deciding whether to cull or not remains highly contentious. The critical question remains whether reductions in animal numbers is essential to avoid progressive deterioration in habitat condition. Alternatively, are episodes of apparent overgrazing an intrinsic and readily reversible feature of vegetation / herbivore interactions? It is widely felt that overgrazing is an intrinsic feature of grazing systems, but a key phrase is episodes of overgrazing. Heavily utilised areas must experience periods of little or no utilisation so that favoured plant species can maximise their reproductive capacity. This scenario of habitat being largely or completely underutilised can no longer occur due to the artificial boundaries currently erected around all Southern African reserves. Game inside reserves or parks cannot migrate when veld condition deteriorates. It is therefore the responsibility of reserve managers to manage both habitat and game populations. Water provision as a management tool Game reserves are (in theory) maintained at ecological as opposed to economic carrying capacity and thus will have a reduced cover of vegetation. The objectives of a game reserve should therefore place emphasis on long term stability and conservation. Overgrazing because of incorrect stocking rates, has been identified as a major cause of accelerated erosion in South Africa. In addition, water availability is a crucial parameter in calculating carrying capacity of a range. Human expansion and fences have, in many cases, prevented seasonal movements of herbivores and it has therefore become necessary to supplement water

12 12 supply to sustain migratory herds throughout the relatively long dry season. Supplementing drinking water for herbivores in conserved areas has, however, long been a contentious issue, but continues to be common practice in veld management in wildlife areas. It has been argued that perennial (yearround) water provision discourages even localised animal migration, with the resulting large congregations of animals around dry season watering points causing overgrazing, trampling and bush encroachment. Excessive waterpoints in conservation areas may have the following adverse effects: favouring water-dependant ungulates and elephants at the expense of rarer species promoting increased predator impacts on prey populations inducing more widespread vegetation impacts worsening animal mortality during droughts decreasing ecosystem stability leading to a loss of biodiversity Additional perennial water provision can inflate a short-term stocking rate beyond an ecologically stable carrying capacity. Ultimately, this leads to an unacceptable rate of soil loss, reduced ecological resilience and reduced habitat diversity. This is since new areas previously inaccessible due to the absence or shortage of drinking water are opened. It has been found that wildebeest and other water dependant species have extended their range because of the provision of perennial water points.

13 13 Conclusion The management of game parks and reserves is a growing discipline. The ideas we accept as fact and readily implement are being challenged on an almost daily basis. Habitat management of savanna ecosystems is a relatively new ecological sub-discipline, with only a 100-year history. We do not know the outcome of the real long term ( year) effects of our efforts today. South Africa today has ± formal conservation areas covering just under 20 million hectares of land, twice as much as there was 50 years ago. With this incredible resurgence of game parks and reserves has come a renewed interest in wildlife and related studies. This obviously results in more in-depth and more focussed research, eventually contributing to better management ideas and techniques for our wildlife ecosystems. This component and the related component of Habitat management (Module # 11), only form the basic foundation of understanding to this developing science. We offer a full Wildlife Management Course.